The invention relates to assigning sending permissions in a telecommunications system.
The general packet radio service GPRS is a new service in the GSM system. The GPRS subnetwork includes a number of packet data service nodes SN, which will be called serving GPRS support nodes SGSN in this application. Each SGSN is connected to a GSM mobile communication network (typically to a base station controller BSC or to a base station BTS in the base station system) to allow the SGSN to provide packet service for mobile data terminals via several base stations, i.e. cells. The intermediate mobile communication network provides a radio interface and packet-switched data transmission between the SGSN and the telecommunication terminal i.e. mobile stations (MS). Different subnetworks are connected to an external data network, e.g. to a public switched data network PSPDN, via GPRS gateway support nodes GGSN. Thus the GPRS service enables packet data transmission between mobile stations and external data networks as the GSM network functions as a radio access network RAN.
In the GPRS network data packets arriving from several users are sent using the same radio channel. Therefore, the unit responsible for timing, i.e. the packet control unit (PCU), has to be able to decide in which radio frame each data packet is sent. The PCU assigns sending permissions for mobile stations sharing a radio timeslot (TS), i.e. tells which mobile station may transmit and when. The packet control unit may be separate from the actual unit providing the radio interface. According to GPRS specifications, PCU may be located at a base station (BTS), a base station controller (BSC) or an SGSN site.
A GPRS block period (BP) is a sequence of four time slots on a PDCH (Packet Data Channel) used to convey one radio block. A radio block is a sequence of four normal bursts carrying one RLC/MAC (Radio Link Control/Medium Access Control) protocol data unit, i.e. a RLC/MAC block which is a data unit exchanged between RLC/MAC entities. The block period is approximately 20 ms corresponding to the period that the transmission of one RLC/MAC block over the Abis interface lasts.
In case dynamic allocation is used for uplink transmission, the PCU assigns a mobile station uplink sending permission by including USF (Uplink State Flag) allocated for the mobile station in downlink RLC/MAC block's MAC header. Mobile stations observe the time slots allocated to their uplink transmission and in case a mobile station recognizes its own USF in downlink RLC/MAC block's MAC header, the mobile station may transmit in the uplink direction on the next block period.
In case fixed allocation is used for uplink transmission, the PCU assigns a mobile station uplink sending permissions in an assignment bitmap. The mobile station does not have to monitor downlink RLC/MAC blocks to find out whether the mobile station may transmit or not. Based on the assigned bitmap the mobile station knows in advance when the mobile station may transmit.
The PCU may also poll a mobile station using a RRBP (Relatively Reserved Block Period) field in the MAC header. For example, in case of downlink transmission, the PCU may include a valid RRBP field in a downlink RLC/MAC data block which causes one uplink block period reservation for the mobile station. In the allocated block period the mobile station shall send a Packet Downlink Ack/Nack message to the PCU and acknowledge received blocks. In the Downlink Ack/Nack message the mobile station may also indicate if it has data to be sent.
In case a mobile station is transferring real-time (RT) traffic, the mobile station needs more sending permissions than a mobile station transferring non-real-time (NRT) traffic in order to meet delay and throughput requirements. An RT mobile station should be able to start transmitting data quite soon (within 40 to 60 ms, for example) after having data at the RLC/MAC layer (e.g. when a user starts to talk).
An RT mobile station having a silent period (i.e. having temporarily no data to transmit e.g. because the user is not talking) is polled in order to learn if the mobile station has data to be transmitted. When the PCU is located apart from a BTS site (in a BSC, for example), the time between sending the polling command and receiving a response from the MS is several block periods (BPs). The delay can be around 8 BPs (160 ms) which consists of the following components: PCU to BTS transmission takes 3 BPs (approximate value, due to e.g. delay caused by buffering inside the BTS), BTS to MS transmission takes 1 BP, MS to BTS transmission takes 1 BP, BTS to PCU transmission takes 3 BPs. Thus the PCU learns only about 8 BPs after polling the mobile station if the mobile station has more data to transmit.
The PCU has currently two ways to schedule (assign a sending permission) a mobile station if the PCU is located apart from a BTS site:
1) The PCU could poll the mobile station every now and then and start assigning sending permissions to the mobile station only after learning the mobile station has more data to transmit. It would take around 12 BPs (8+4) before the mobile station would be able to transmit data at normal phase.
2) The PCU could assign a sending permission to the mobile station e.g. every second BP. When the PCU learns the mobile station has more data to transmit, the PCU starts assigning sending permissions to the mobile station more frequently (e.g. at every BP). Before the mobile station receives sending permissions frequently enough, the mobile station may use the already sent polling permissions to start sending data.
The alternative 1 is probably much too slow and thus would give unacceptable transmission delay for an RT mobile station. The alternative 2 may give acceptable service for the RT mobile station in case the PCU polls the mobile station frequently enough. However, the polling sending permissions the RT mobile station uses to start its transmission may not be enough to satisfy the delay and throughput characteristics. Furthermore, sending permissions are wasted if the mobile station is polled frequently and it has nothing to send.